1. Field of the Invention
The present invention relates to the elimination of byproduct aqueous effluents comprising hydroxynitroaromatic type compounds, and, more especially, to the removal of byproduct aqueous effluents which result during the preparation and purification of nitroaromatic compounds via reaction of aromatic compounds with nitric acid in the presence of sulfuric acid.
2. Description of the Prior Art
It is known to this art that, following nitration of aromatic compounds, the reaction medium is treated to separate the desired nitroaromatic compounds thus produced from the mineral acids employed as reactants and as catalyst, then from the byproducts formed.
This operation normally includes a number of successive wash steps, each followed by a decantation/separation cycle for the organic and aqueous phases. Thus, the reaction medium is first contacted with water to recover the nitric and sulfuric acids. One or more washes are then carried out using water containing a neutralizing agent to eliminate the byproducts.
By the term "byproducts" are intended hydroxyaromatic compounds containing one, two or three nitro groups (such as nitrophenols and nitrocresols). These compounds are the most objectionable, as they are known to poison the catalysts used in the final hydrogenation reaction to which the nitroaromatic compounds may be subjected. This reaction reduces the nitro groups to amine groups and the resulting amines can then be used as reactants for the production of compounds such as isocyanates.
The term "byproducts" also comprehends aromatic carboxylic acid compounds containing one, two or three nitro groups and carboxylic acids containing one to six carbon atoms, such as formic, acetic or oxalic acid.
These byproducts, a representative number of which having been indicated above, are designated either as byproducts or as hydroxynitroaromatic compounds in the description that follows.
Treatment and/or elimination of the aqueous phase recovered after the various washing steps, corresponding to separation of the nitroaromatic compounds from the byproducts formed, presents a serious disadvantage as the quantity of these byproducts is considered to be too great. Consequently, the effluents cannot be discharged without prior treatment.
One possible solution to this problem is to collect the effluents and incinerate them, optionally following concentration thereof. However, this technique is not economically viable because the cost of both concentrating the effluents and then incinerating them is too high. In addition, this technique does not eliminate the problem of discharging the inorganic compounds used, particularly for the neutralization step.
Another possible solution is to extract the byproducts, principally the hydroxynitroaromatic compounds, by means of a suitable solvent, then concentrating and incinerating the extracted species. However, this operation is also of no economic interest because of the multiplicity of steps (extraction, concentration) and the requirement for additional reagents.
Chemical or biochemical treatment of aqueous effluents prior to their discharge to destroy byproducts present in the aqueous phase is also known to this art. These methods, however, can also be very costly and entail an extra step in the sequence. In addition, a very large amount of water is discharged, and even if the hydroxynitroaromatic compounds have been removed, the water nevertheless contains a significant fraction of neutralizing agent which is lost and which should be destroyed before discharging the effluent to waste.
It will thus be seen that no efficient and economically advantageous treatment currently exists to combat the problem of waste water emanating from an aromatic compound nitration process.